Historical imagery

Raster data

• Grid of cells that form a continuous surface
• Each grid cell contains a value that represents some real-world phenomena (e.g. elevation, land use)

Quantitative and qualitative rasters

Raster data resolution

• Cell dimension: the size of each grid cell (e.g. 30 meters by 30 meters)
• Resolution: the relative size of the grid cell

Types of raster data

• ArcGIS and other GIS packages can handle a variety of raster formats, including:
• Image files (JPEG, TIFF, BMP, PNG, etc.)
• Geo-enhanced image files (GeoTIFF, GeoJPEG)
• ESRI GRID, ESRI ASCII GRID
• Remotely sensed imagery (e.g. ERDAS Imagine .img files)
• Many more.

Raster data and cell sizes

3D coordinate systems

• Z-coordinates: vertical measurements (elevation), or measurement of magnitude

Vertical datums

• Reference for measuring heights/elevation
• Common vertical datums:
• Mean sea level
• The geoid (orthometric heights)

Digital Elevation Models (DEMs)

• Raster representation of a terrain’s (commonly the Earth’s) surface

How DEMs are produced

• Original elevation measurements taken by field surveying
• Additional information supplied by:
• Analysis of remotely sensed imagery / photogrammetry
• Laser technology (Light Detection and Ranging, or LiDAR)
• USGS National Map Viewer: Portal for downloading DEMs

DEMs in ArcMap

Surface (terrain) analysis

• Used to model the topographic characteristics of a given landscape
• Applications:
• Shaded relief mapping
• Construction and transportation planning
• Hydrologic modeling (flood zones, river flow, watershed boundary delineation)

Slope

• Definition: how elevation changes with horizontal position (rise over run!)
• In a GIS, slope is often measured in degrees

How slope is calculated

Aspect

• The steepest downhill direction at a given point; can be thought of as the direction of the slope

Hillshading

• Representation of three-dimensional characteristics of a two-dimensional surface
• Used to produce shaded relief maps

Surface analysis in ArcMap

Contours

• Lines that connect cells of equal value in a raster (e.g. isolines)
• Applications: elevation, temperature, pollution, etc.

Viewshed analysis

• Viewshed: the cells in a raster that can be viewed from a given point(s)
• Lines of sight calculated for points based on a DEM

3D GIS concepts

• Extrusion: vertical stretching of a 2D shape for display in 3D, based on some attribute or value

• Base heights: how elevation/height values are defined within a three-dimensional GIS view

• Drape: display of a layer in three dimensions given the defined base heights

3D visualization in ArcScene